KConfig: Convert to SPDX license statements
Summary:
Convert license headers to SPDX statements and add
license files as required by REUSE specification.
Reviewers: cgiboudeaux
Reviewed By: cgiboudeaux
Subscribers: ognarb, cgiboudeaux, kde-frameworks-devel
Tags: Frameworks
Maniphest Tasks: T11550
Differential Revision: https://phabricator.kde.org/D27601